3-phase medium voltage cables are used in numerous applications for distributing electrical energy from generators or substations to consumers. 3-phase cables are typically used as a distribution network to feed electrical motors or power electronic equipment. In such a 3-phase cable network there is a need to measure the current being distributed to the consumer. The current must be measured to determine and log the power being transmitted to each consumer and to be able to react in case of malfunctions or failures such as a short-circuit occurring in the distribution network. Historically, cable networks were used for low voltage and medium voltage applications while overhead lines were used for high voltage applications due to the increased insulation requirements at higher voltages. However, overhead lines have the drawback of being more exposed to environmental influences such as storms, blizzards and the like. New developments of high voltage insulation materials and a greater dependence on interruption-free distribution networks lead to high voltage cables replacing overhead lines.
A 3-phase cable typically has the individual 3-phase conductors encapsulated inside a common insulator. Each phase conductor is individually insulated in relation to the other phase conductors inside the cable by being embedded in a high voltage insulating material. To achieve the efficient and well-known round shape of the cable, typically each phase conductor accommodates a section-shaped space inside the cable. A shield of conductive material typically encloses the insulated conductors. The shield is typically grounded and primarily used as a barrier for protecting the 3 phase conductors from damage and for protecting persons from electric shock in case of damage on the insulation. The shield is also used as a return path for the current, if needed.
To measure the current in a 3-phase cable, typically the cable needs to be interrupted and a measurement device such as a Rogowski coil or current transformer has to be attached to each phase separately. This is very cumbersome and also a security risk, since the outer insulation and the shield must be removed from the cable. For very high voltages it is not always possible to separate the phases, or the measurement equipment needed would occupy a very large space due to the large insulation distances needed. It would therefore be an advantage to be able to measure all 3-phase currents directly on the 3-phase cable.
An advantageous technology for measuring the current in a single phase has been described in the European patent application 08388018.7 filed on 30 May 2008, which has now been published as EP 2128631. In the above patent application a Faraday optical current sensor is used to measure the current in a single phase of a power line.
Faraday optical current sensors may be used for measuring a current in a nearby conductor. Faraday optical current sensors rely on the Faraday effect. The Faraday effect states that the rotation of a polarized light beam is proportional to the magnetic field component in the direction of the beam. A charge moving inside a conductor will produce a circular magnetic field around the conductor. Thus, by placing a Faraday optical current sensor parallel to the direction of the magnetic field lines the magnitude of the current may be measured.
Using a Faraday optical current sensor provides many advantages compared to conventional current measurement equipment such as current transformers, Rogowski coils and the like. One of the most important advantages is the fact that the Faraday optical current sensor may be constructed entirely from dielectric materials. This is especially important for high voltage applications since it gives the Faraday optical current sensor a substantial immunity against electric field disturbances. Another important advantage of the Faraday optical current sensor is that since it is galvanic and separated from the conductor, it does not influence the current in the conductor in any way. This almost eliminates the risk of a short-circuit of the conductor through the measurement system. One example of such a Faraday optical current sensor is the DISCOS® Opti module produced by the applicant company and described in U.S. Pat. No. 7,068,025, to which reference is made and which is hereby incorporated in the present specification by reference. Further examples may be found in the PCT applications WO 2006/053567 and WO 04/099798, which are both hereby incorporated in the present application by reference.
A Faraday optical current sensor comprises a sensor element being a magneto-optical part sensitive to magnetic fields and typically formed as a diamagnetic rod, fibre or similar made of a material exhibiting a high Faraday effect. This is understood to mean a material having a high Verdet constant. The Verdet constant is the proportionality constant of the Faraday effect and varies considerably between different materials. Tabulated values exist for various suitable materials. The angle of rotation of the polarized light may be described by the following formula:β=V×B×d 
where β is the angle of rotation, d is the length of the path where magnetic field and light interact, B is the magnetic field in the direction of the light propagation and V is the Verdet constant. The magnetic field at a certain location outside a conductor may be calculated by using the following well-known formula:
  B  =                    μ        0            ⁢      I              2      ⁢      π      ⁢                          ⁢      r      
where B is the magnetic field, μ0 is the magnetic constant, I is the current and r is the distance from the conductor.
The magneto-optical part may be supplied with polarized light from a light source such as a lamp or LED emitting linear polarized light in a specific wavelength. The light source may comprise a polarized filter for generating light with a specific linear polarisation. The light exiting the magneto-optical part may be detected and preferably converted to an electrical signal by a detection unit. The detection unit detects the rotation of the polarized light exiting the magneto-optical part. A control unit may evaluate the signal from the detection unit; perform the necessary error corrections and calculations to determine the current in the conductor. Possible sources of errors include sensor position in relation to the power line, optical noise, magnetic noise, transition effects when light enters and exits different optical media and temperature effects. The Faraday optical current sensor is preferably calibrated before use, e.g. by comparing measurement results to conventional current measurement equipment. Conventional current measurement equipment may comprise e.g. a current transformer. After calibration the Faraday optical current sensor may replace conventional measurement equipment for monitoring currents in the conductor. The Faraday optical current sensor may also be used to detect fault currents such as short-circuit currents and report such occurrences to a safety system, which may in turn activate the relevant circuit breakers and backup systems to avoid damage to other equipment in the power distribution network.
The magneto-optical part, the light source and the detection unit are preferably connected via an optical conduit such as an optical fibre. Optical fibres provide a substantial amount of flexibility and allow light to travel long distances without considerable losses in light intensity. However, it is important to be aware of the limits in flexibility of optical fibres. Optical fibres may fail due to being broken, damaged or deformed if they are bent beyond their flexibility limit. A failure in the optical fibre due to excessive bending will typically permanently make it unusable for conducting light. Typical optical fibres may be bent considerably less than electrical cables.
For better handling and protection against damage and ambient light sources, the magneto-optical part as well as the junctions with the ends of the optical conduits are encapsulated by a small cylindrical housing. All of the above features of the optical Faraday optical current sensors make a broad range of new measurement positions feasible.
Since optical sensors may be constructed by using only dielectric materials, the sensors may be positioned in locations where other sensors, i.e. sensors comprising conductive materials, are not suitable. Such locations include places subject to high electrical fields, which are common in high current and high voltage engineering. Additionally, the Faraday optical current sensors are very compact and light since they do not contain any metal parts. The magneto-optical part for high voltage and high current applications may be realized having dimensions in the mm range, compared to current transformers for high voltage application often having dimensions of several meters due to the safety distances needed for high voltage applications. Such large current transformers will be unable to measure the current accurately directly on a 3-phase high voltage cable. There is therefore a need for current measurement systems and methods for measuring the current directly on a 3-phase cable using a Faraday optical current sensor.